1. Field of the Invention
This invention relates to a photoelectric position detector in which a scanning movement of a scanning phase grating relative to a scale member which is provided with a reflective phase grating is detected, light emitted by at least one light source is diffracted by the scanning grating and as it is reflected by the scale member and then again by the scanning grating to form an interference pattern and only groups belonging to said interference pattern and being of a predetermined diffraction order are detected by respective photodetectors and are converted by them to phase-displaced photodetector signals, which are cyclically changed as the scanning grating is moved relative to the scale member.
2. Description of the Prior Art
The basic principle of such position detectors is known from British Patent Specification 1,474,049. In those known detectors, the zero order group, i.e., the undiffracted group of the diffraction image and groups thereof which have positive and negative phase displacements of the same magnitude are received by the photodetectors and the resulting photodetector signals can be evaluated for a detection of the direction of movement of the scanning grating relative to the reflective phase grating of the scale member, and of the extent of the movement of the scanning grating. In accordance with Published German Application 23 16 248 a similar system comprises three photodetectors, which detect the zero-order group and the first-order positive and negative groups of the diffraction pattern. A special embodiment of the last-mentioned system is known from EP-B-0 163 362 and comprises a special scanning grating, in which the ratio of the rib and groove widths differs from unity and is, e.g., of an order of 1:3, and the rib height is correspondingly determined and the photodetectors are so arranged that the sensitivity to a change of the phase displacement by changes of the distance between the gratings and by a reception of the zero-order and positive and negative first-order diffraction groups is reduced because three photodetector signals are generated, which have a phase displacement which is as close as possible to 120.degree.. Whereas the desired phase displacement can substantially be achieved by the last-mentioned position detector it has essential disadvantages just as all other known position detectors of that kind. In the first place the absolute amplitude or the absolute change of the amplitude of the photodetector signals may change in dependence on the illuminance, on the distance between the gratings and on the optical losses involved in the measuring operation and for this reason the photodetector signals must be normalized before they can be evaluated. Besides, measured-value signals which can be used for a measurement of lengths and angles cannot directly be derived from three signals displaced 120.degree. in phase but said three signals must be shaped and converted by a computation to two sinusoidal analog signals, which are displaced 90.degree. in phase and from which countable digital signals can be derived by methods which are conventionally employed in normal incremental measuring systems and can be used to control bidirectional counters and subsequently for an indication of the linear or rotational movement which has been performed.
In position detectors of a different kind it is shown that interference caused by internal reflection can be avoided in that at least one transparent phase grating is provided and the diffracted partial beams are directed back to photodetectors by an optical deflector through the grating or gratings at a distance from the region on which the light is initially incident. In such position detectors a laser is preferably used as a light source. Thereafter, zero-order diffraction patterns and positive and negative n-th-order diffraction patterns are detected and used for a generation of three photodetector signals, which are displaced 120.degree. in phase and from which measured-value signals are derived by the above-mentioned shaping and computational conversion. It is known from EP-A-0 387 481 to provide only one phase grating on the scale member, to use transmitted light and to direct two coherent partial light beams onto the phase grating at angles to the normal so that said partial beams are diffracted by the grating and interfere with each other and photodetector signals can be derived from the interference pattern by photodetectors of a measuring apparatus. In systems comprising two gratings it is also known so to direct the light that it is incident on the first grating at a given angle so that the direction of movement can be more readily detected by the measurement and sufficiently differentiated diffraction patterns are generated for being detected by the photodetectors.
From EP-A-0 223 009 it is known to provide on the scanning member two phase gratings, which are arranged one behind the other and offset in phase by one-fourth of the grating constant and which have a rib-groove width ratio differing from 1:1. In that case the measuring range extends over a relatively large length of the scale member and the arrangement is expensive but affords the advantage that a normalization of signals can be effected in that the signals generated by the photodetectors are combined. Three photodetectors are provided for each grating and are used to detect the light beams diffracted onto said photodetectors.